In 2009, Engineering Control Limited (ECL) was contracted to upgrade the Mokau station using advanced process and safety control solutions from Rockwell Automation.
The Mokau Compressor Station on New Zealand's North Island compresses gas running through the 307 km Maui Pipeline, which is operated by Vector Gas.
The primary function of the Mokau station is to ‘line-pack’ gas to the north. It comprises two centrifugal compressor-sets, each powered by a gas turbine engine. The compressors are designed to increase the gas pressure in excess of 50 bar, which enables the gas to flow north without the need for additional pumping stations.
Control under pressure
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Prior to the upgrade, three controllers were needed to operate the Mokau station — the station controller, and a controller for each of the gas turbines. The station controller supervises the entire station, communicating with the gas turbine controllers, and managing the five operating modes – run, ready, inhibit, emergency shut down (ESD), and backflow. In an emergency, the station controller initiates ESD mode — stopping the compressors, depressurising the station pipework, and allowing gas in the Maui Pipeline to bypass the station.
ECL Senior Control Systems Engineer Peter Huitema says “The station controller had reached the end of its design life, it was tough to get spares, and licensing issues made it difficult to get online to diagnose problems.”
Upgrading automation
Vector Gas had previously upgraded the gas turbine controllers to Rockwell Automation’s Allen-Bradley ControlLogix platform, and the company decided to use the same technology to replace the station controller. The aim was to enhance the automation control functionality and improve the communications between controllers, while replicating the base functionality of the legacy station controller.
The project also involved upgrades to the motor control centre, incorporating the replacement of the legacy drives for the gas and oil coolers with six variable-speed drives fitted with soft starts, which significantly increased overall operational efficiency. Similarly, the legacy overload relays for the turbine enclosure fans were replaced with new overload relays that facilitate the transmission of operational data and status information.
Vector Gas Instrument and Electrical Engineer Alan Taylor says “This upgrade has afforded us unprecedented levels of communication, and naturally helps improve the reliability of the whole station.”
The project also enabled Vector Gas to increase the safety integrity level (SIL) of the control system to SIL2.
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